Apparatus for regulation of chip removal in teeth-generating grinding of gear wheels

ABSTRACT

An intermittent tooth generating grinding machine for gear wheels has a bed and a table mounted for translational movement on the bed. Such translational movement is produced by turning a screw which is located completely outside the gear box and is driven therefrom by connecting gearing. The length of the screw, other than the portion mounted in its bearings and the driving gearing, is substantially equal to the translational movement of the table. The screw carries a nut. In one form, the screw is slidable on the bed between adjustable stop members and the nut is fixed to the table. In a second form, the screw is fixed on the bed, the stop members are fixed on the table and the nut is engaged between the stop members. In the third form, the screw is fixed on the table and the stop members on the bed and the nut moves between the stop members.

United States Patent [191 I Hiifler Inventor:

Filed: June 9, 1971 Appl. No.2 151,380

Foreign Application Priority Data June 15, 1970 Germany P 20 29 359.8Mar. 24, 1971 Germany P 21 14 151.5

US. Cl. 51/95 GH, 51/165.8, 51/232 Int. Cl B24b 19/00 Field of Search51/123 G, 95, 165.8,

References Cited UNITED STATES PATENTS 2/1933 Raule ..51/232 11/1936Schicht ..5l/232 4/1915 Drummond 51/123 G 1 Oct. 9, 1973 3,593,4637/1971 I Uhterwoldt 51/l65.8

Primary Examiner-Harold D. Whitehead Attorney-Jennings Bailey, Jr.

1 [5 7] ABSTRACT An intermittent tooth generating grinding machine forgear wheels has a bed and a table mounted for translational movement onthe bed. Such translational movement is produced by turning a screwwhich is located completely outside the gear box and is driven therefromby connecting gearing. The length of the screw, other than the portionmounted in its bearings and the driving gearing, is substantially equalto the translational movement of the table. The screw carries a nut. Inone form, the screw is slidable on the bed between adjustable stopmembers and the nut is fixed to the table. In a second form, the screwis fixed on the bed, the stop members are fixed on the table and the nutis engaged between the stop members. In the third form, the screw isfixed on the table and the stop members on the bed and the nut movesbetween the stop members.

12 Claims, 8 Drawing Figures PAIENTEUHBT 9mm sum 2 or 7 I N VEN TORPAIENTEDIIBI elm 'LTSGLSQQ SHEEI BM 7 FIG. 4

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sum n or 7 IN V EN TOR PATENTED 9 I 75 SHEET 70F T n VOOOOOOOOOOWM 2 mm2 llllll IILII rlliillllll Q F wdE IN VEN TOR l APPARATUS FOR REGULATIONOF CHIP REMOVAL IN TEETH-GENERATING GRINDING OF GEAR WHEELS BACKGROUNDOF THE INVENTION 1. Field of the Invention This invention relates toapparatus for regulation of chip removal in intermittenttooth-generating grinding of gear wheels in which a table for theworkpiece is operatively connected with a nut on a screw which isrotatable periodically in opposite directions for imparting to theworkpiece table the translational component of translational androtational components of the generating movement of the workpiecerelative to a grinding wheel, and in which for regulation of chipremoval the nut can execute an axial displacement limited by twoadjustable stops.

2. The Prior Art ln such known apparatus the screw can execute an axialdisplacement, limited by the stops, for regulation of chip removal. Thestops are provided on a spindle of the screw situated in the gearcase ofthe grinding ma chine and comprise bushings which are fast on thespindle of the screw and each of which bears with an annular face,through aninterposed roller thrust bearing and an intermediate bushing,on a corresponding annular face of an adjusting bushing mounted on thespindle. Each adjusting bushing is movable along the spindle by means ofa screw thread on the bushing in engagement with an internally threadedworm wheel seated on it concentrically with the spindle and bearingthrough a thrust bearing on an intermediate wall of the gearcase, theworm wheel meshing with an actuating worm therer for which is accessiblefrom the exterior of the gearcase. Adjustment of the axial shiftdistance permitted to the rotatable screw is effected by operation ofeach worm to displace the associated adjusting bushing by the desiredamount of infeed can be adjusted and read off from the respectiveposition of the infeed worm separately for the left and the right toothflanks to be ground. The two adjusting bushings are operativealternately, for limiting the axial displacement of the rotatable screw,according to the direction in which the workpiece table is being movedby the screw in the course of the periodic movement of the table backand forth.

A disadvantage of this known arrangement is the great difficulty inkeeping indexing and transition errors within tolerable limits in theproduction of precision gear wheels. Thus, inter alia, the lack ofprecision of the flatness of the annular stop faces of the adjustingbushings at all adjusted positions thereof is a constant source oferror. With a bushing which is moved along the screw spindle rotatablyby screw threads, it is in fact not technically possible to obtain anaccuracy of flat-- ness of a few u m on its annular stop surface for anydesired angular positions of the bushing, even if the bushing is screwedinto the internal thread of its worm wheel during the grinding of itsannular face. Because of these errors of flatness, dependent upon theangular position of the adjusting bushing, in place of the entireannular stop face only a partial zone thereof comes into abutment withthe intermediate bushing, so that the axial displacement thereof isdetermined not solely in accordance with the exactly reproduciblerotating movement of the worm wheel, but also according to theuncontrollable error in flatness of the annular stop surface of theadjusting bushing.

A further source of error arises from the yieldability of theintermediate wall of the gearcase which has to absorb the high axialforces of the screw. For reasons of manufacturing technique the gearcasecannot be case on to the machine housing and therefore is greatlyendangered by the periodical alternating thrusts of the axially shiftingscrew.

Another source of error lies in the gradual heating and consequentialthermal expansion of the screw, which leads to variations in its length,with the result that, as with the other two sources of errorabovementioned, differences of position of the workpiece table and thusof the tooth flanks to be ground occur in relation to the grindingwheel. Since the heating increases with the grinding time, this meansthat between the beginning and the end of the grinding a difference ofposition occurs which leads to transition errors between the tooth firstground and that last ground. The longer the grinding time, the greaterdoes this transition error become, so that correspondingly great errorsoccur in the grinding of gear wheels with great tooth width.

Summary of the lnvention An object of the present invention is toprovide improved apparatus of the kind initially mentioned, ofsubstnatially improved accuracy especially in reducing the influence ofthe abovementioned sources of error. Another object is to accomplishsuch improvement with apparatus of simple and economic construction.

This invention accordingly consists in apparatus for regulation of chipremoval in intermittent teethgenerating grinding of gear wheels in agrinding machine in which a table for the workpiece is operativelyconnected with a nut on a rotatable screw driven from a drive gearingwhich is housed in a gearcase of the machine and is adapted to rotatethe screw periodically in opposite directions for imparting to the tablethe translational component of translational and rotational componentsof the generating movement of the workpiece relative to a grindingwheel, and in which for regulation of chip removal the nut is axiallydisplaceable between limits determined by two adjustable stops which arearranged apart from the screw and on mountings independent of thegearcase, the length of the screw being dimensioned substantiallyaccording to the amplitude of the translational component, and the screwbeing arranged -and mounted totally outside the gearcase and with adriving connection between the drive gearing and the screw.

in one form of the invention the nut is mounted on the workpiece tablein fixed position relative thereto,-

while the stops are anchored on the machine bed in alignment with theends of the screw which is mounted on the machine bed and is axiallydisplaceable between the stops.

Alternatively, in a second form of the invention the nut may be mountedon the workpiece table so as to be axially displaceable relativethereto, the stops anchored on the workpiece table, and the screwmounted on the machine bed.

As a further alternative, in a third form of the invention the screw maybe mounted on the workpiece table,

the stops anchored on the machine bed, and the nut axially displaceablebetween the stops.

In the first and second mentioned forms of the invention the drivegearing (with its gearcase) for the screw is mounted on the machine bed,whereas in the third mentioned form the drive gearing and gearcase maybe mounted on or attached to the workpiece table and thus take part inits translational movement.

With stops aligned and cooperating with the screw ends, the stops haveonly a relatively slight radial extent, so that unavoidable errors offlatness of the stops can have no appreciable effect, while with stopsarranged to cooperate with the nut the problem of errors of flatness iscompletely excluded, because flatness between the stops and the nut isnot important. While in the second and third mentioned forms of theinvention axial displacement of the nut is limited directly by thestops, in the first mentioned form of the invention this is effectedindirectly through the axial displaceability of the screw.

Since in this invention the anchoring of the stops is independent of therelatively yieldable gearcase, being effected on the substantially morestable machine bed or workpiece table, yielding of the stops under theaxial force exerted alternately upon them is avoided. Also, with thearrangement of the screw totally outside the gearcase, so that it nolonger needs to be extended into the drive gearing but is driventherefrom by an intermediate driving connection such as an intermediatewheel arranged outside the gearcase, the length of the screw andcorrespondingly the error-producing variation in length thereof due toheating can be reduced substantially, in practice to be onlyapproximately half as great as in the prior art arrangement. Thetransition error introduced by such additional intermediate wheel is ofinconsequential effect since it is converted by the nut into atranslational movement with a reduction of 40:1 in practice; that is tosay, if a total indexing error of 10 p. m is tolerated, an error of onlym occurs in the translational movement of the workpiece table. Since ingenerating grinding machines for the grinding of large gear wheels thescrew may be up to 5 meters in length, the substantial shortening of thescrew effected in this invention also reduces the risk of the screwssagging under its own weight or as a result of the axial force acting onit.

Individual adjustment of each stop may be effected by means of aworm-actuated worm wheel turning the stop which has an external screwthread engaged in a fixed stand. Alternatively, the adjustment may beeffected by means of a spring-loaded wedge which is displaceable along acounter-piece in an oblique direction relative to the axis of the stop.The wedge may be displaceable by means of a worm-actuated worm wheelturning a member engaging the wedge which has an external screw threadengaged in a fixed stand.

The stops may be formed as bolts with domed abutment faces, and, whenthe stops cooperate with the ends of the screw as above described, thetwo end faces of the screw preferably are provided with hardenedinserts. These measures further reduce the effect of errors of flatnessof the stops when their adjustment is effected by a worm wheel and screwthreading as above mentioned.

It is advantageous to provide for each stop an associated displacementpickup having a feeler member engaging the stop for sensing thedisplacement thereof, each pickup being connected to an individualindicator for indicating the adjusted position of the associated stop,and both pickups being connected to a differential indicator forindicating any differential which may occur between the adjustedpositions of the stops.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is explained furtherbelow by reference to embodiments illustrated as examples. In theaccompanying drawings:

FIG. 1 is a diagrammatic plan representation of the prior art form ofworkpiece table drive of a gear generating grinding machine;

FIG. 2 is a section on a larger scale of the portion of FIG. 1 markedFIG. 2" and shows the prior art arrangement of stops for the limitationof the axial displacement of the screw;

FIG. 3 is a diagrammatic plan representation, corresponding to FIG. 1,but of a workpiece table drive according to one embodiment of theinvention;

FIG. 4 is a side elevation, partially in section, of the drivearrangement according to FIG. 3 shown on a larger scale;

FIG. 5 shows a modification of the stop adjusting means shown in FIG. 4;

FIG. 6 shows a second embodiment of the invention depicted in a mannercorresponding to FIG. 4;

FIG. 7 is a diagram of the indication of the cutting infeed; and

FIG. 8 shows a third embodiment of the invention, again depicted in amanner corresponding to FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Corresponding parts in thevarious Figures are denoted by the same reference numerals.

Referring to FIG. 1, a gear generating grinding machine has a workpiecetable 1 for carrying a gear wheel workpiece 2 to be ground and which iscaused to execute in each case a generating movement relative to agrinding wheel 3. The workpiece 2 is secured to the table 1 by means ofa clamp 4. The workpiece table 1 is displaceable rectilinearly onlongitudinal guides 5 to impart a translational component of thegenerating movement to the workpiece 2. The displacement of theworkpiece table is effected by a screw 6 engaged in a nut 6 carried bythe table 1, the screw being rotatably driven periodically in themathematically positive and negative directions of rotation, from acrank gearing situated in a gearcase 7 arranged on the machine bed. Theproduction of a rotational component of the generating movement of theworkpiece is effected in the usual manner through pitch blocks orthrough pitch change wheels.

To allow an infeed movement of the workpiece relative to the grindingwheel during the grinding of the gear wheel, it is usual to vary theassociation between the rotational and translational components ofmovement of the workpiece. When pitch change wheels are used for thegeneration of the rotational component, this variation can be effectedfor example by permitting the screw a predetermined and adjustable freestroke in the axial direction so that the tooth flank to be ground ispivoted towards the grinding wheel face, without a correspondingdisplacement of the workpiece table taking place, until the free strokefor the axial displacement of the screw is completed.

The prior art arrangement of stops for the limitation of the axialdisplacement of the screw is shown in FIG.

I and 13 on the spindle, respectively. The bushings 12 and 13 are eachnon-rotatably but axially displaceably mounted in a housing wall andbear respectively on adjusting bushines l4 and on the spindle whichconstitute the actual stops. Bushings 14 and 15 are nonrotatably mountedbut are movable axially on the spindle by a screw thread provided ontheirexte'rnal cylindrical surfaces engaging an internal screw thread inworm wheels 16 and 17 which are rotatable by means of worms 19 and 20accessible from the outside of the gearcase 7. The screw 6 is driven bythe crank gearing in gearcase 7 through a gear wheel 18 which is splinedon the screw spindle to permit relative axial displacement between thegear wheel and the spindle. Gear wheel 18 bears through thrust bearings21 and 22 on the housing 23.

Thus by turning the worms l9 and 20 the play between the clamp sleeves 8and 9 and the adjusting bushings 14 and 15 can be adjusted. As a resultof this play, on a reversal of rotation of the gear wheel 18 by thecrank gearing, first an axial movement of the screw 6 takes place, whilethe screw turns in the nut and workpiece table 1 remains at rest becauseof its substantially greater inertia to changes of movement,untilengagement of collar 8 or 9 with bushing 14 or 15 blocks furtheraxial displacement of the screw 6 after which continued rotation of thescrew results in movement of the table 5. Therefore the desired amountof infeed (.1: or x can be adjusted separately for each tooth flank bymeans of the two worm wheels 16 and 17.

In FIGS. 3 and 4 a drive arrangement according to the invention isillustrated, according to which the drive of the gear wheel 18 and thusof the screw 6, which turns in nut 25, takes place, not within thegearcase 7 as in FIG. 2, but at a location outside the gearcase by meansof a shaft 24 (FIG. 3) protruding from the gearcase, and carrying a gearmeshing with gear 18, so that the screw 6 is now shortenedsubstantially. As FIG. 4 shows, the adjustable stop members for thelimitation of the axial displacement of the screw 6 are separate fromthe screw 6 and are constituted by bolts 28 and 29 threadedly mounted inalignment with the ends of the screw 6 in housings 30 and 31 which arean- .chored by bolts on the machine bed 27 to which the radial bearings26 of the screw 6 are also secured. The bolts 28 and 29 are turned andthus axially displaced in their housings by means of wonn wheels 32, 33and actuating worms 34, 35 for the worm wheels. Stops 28 and 29 arekeyed to worm wheels 32 and 33 so as to be slidable but non-rotatablewith respect thereto. The

ends of the bolts which face the ends of screw 6 are provided'with domedabutment-faces in order to reduce to a minimum the errors of flatness ofthose faces over the range of adjustment of the bolts 28 and 29.

Since the axial displacements of the bolts 28 and 29 correspond with theamounts of infeed on the respective tooth flanks, electric pickups 36,37 are arranged to sense the adjusting movements of the bolts 28 and 29respectively and to transmit signals responsively to indicatorinstruments 38, 39 and 40 depicted in FIG. 7. In this case each of thetwo instruments 38 and 39 is connected in circuit with one of thepickups to indicate the amount of infeed separately for each toothflank,

while the instrument 40 is of a differential type connected in circuitwith both pickups to indicate any differences of the amounts of infeedbetween the one tooth flank and the other, so that symmetrical grindingof a tooth gap is facilitated.

The infeed may be effected by motor means, and push buttons 41 to 44 areprovided for switching on and off and reversing the direction ofrotation of two infeed motors (not shown) driving the worms 34 and 35respectively.

In a modification shown in FIG. 5 for the control of the stops 28' and29', the actuation of the bolt 28' (or 29') which is guided in a sleevebearing 45 is effected by means of a wedge 46 which slides along acounterpiece 47 and is displaceable by an infeed bolt 48 against theaction of a spring 49. The infeed bolt 48 is displaceable in the manneras already described by rotation of a worm 51 and a worm wheel 50. Theassociated pickup 36' (or 37') is in this case arranged to sense theadjusting movements of the infeed bolt 48.

To reduce wear, a hardened, ground and very finely lapped insert 54 issecured in each end face of the screw 6. The domed abutment face of eachof the bolts 28' and 29 is likewise hardened and lapped.

In the embodiment of the invention depicted in FIG. 6, regulation ofchip removal is effected not by axial displacement of the nut by meansof an axially displaceable screw, but by axial mobility of the nut 25relative to the workpiece table 1. In FIG. 6 the screw 6 is mounted withradial bearings 26 which are secured on the machine bed 27, and thescrew is secured against axial displacement by a double-acting thrustbearing 52. The cooperating nut 25 is axially displaceably mounted inlongitudinal guides 53 on the workpiece table 1, its axial mobilitybeing limited by stop bolts 28 and 29 of the nature already described,with which electric pickups 36 and 37 are associated as before forindicating the adjustments of the stops. In this embodiment, however,the bolts 28, 29 and pickups 36, 37 are secured to the workpiecetable 1. The screw 6 is driven through the gear wheel 18, and uponreversal of rotation, as a result of the inertia of the workpiece table1, the nut 25 applies itself against the abutment face of the bolt 28 or29 according to the direction of rotation of the screw.

In FIG. 8 an embodiment of the invention is illustrated which may beregarded as a reversal of the ar-. rangement according to FIG. 6. InFIG. 8 the screw 6 is mounted on the workpiece table 1 with radialbearings 26 and axial thrust bearings 52, while the nut 25 is axiallydisplaceable between stop bolts 28 and 29 which are mounted in housingssecured on the machine bed 27. The guidance of the nut 25 relative tothe machine bed 27 is effected through longitudinal guides 52.

The embodiment of FIG. 8 additionally has the advantage that a drivegearing 55 for the screw 6 can be arranged on the workpiece table I indirect driving iting movement of the screw part with respect to the bedpart, while the nut part is secured to the table part.

In the form of FIG. 6, the screw part is fixed on the bed part and thestop members are fixed on the table part, where they limit the movementof the nut part.

In the form of FIG. 8, the screw part is fixed on the table part and theadjustable stop members are on the bed part where they limit themovement of the nut part.

I claim:

I. In an intermittent tooth-generating grinding machine for gear wheelshaving a bed part and a table part and means mounting the table part fortranslational movement with respect to the bed part, a screw element,means mounting the screw element on one of said bed and table parts forturning movement with respect to such part, a nut element threadedlyengaged on said screw element, and a pair of adjustable stop membersmounted on one of the table and bed parts in the path of movement of oneof the screw and nut elements for limiting movement of such element withrespect to the part on which the stop members are mounted, one of thescrew and nut elements being fixed to one of the bed and table parts andthe other of the elements being movable between the stop members forengagement therewith, means fixedly holding the nut element nonrotatablyabout the axis of the screw element, whereby turning of the screwelement produces movement of one of the screw and nut elements when suchelement is out of engagement with the stop members without producingmovement of the table and produces movement of the table after suchengagement, a gearcase including gearing carried by one of the bed andtable parts, said screw element being located completely outside thegearcase, and means operatively connecting the screw element to thegearing to be driven thereby alternately in opposite directions, saidgearcase being mounted on the other of the bed and table parts from thaton which the stop members are mounted.

2. In a machine as claimed in claim 1, the screw part being mounted onthe bed part for axial movement with respect thereto, the stop membersbeing mounted on the bed part and limiting such movement of the screwpart, and the nut part being fixed to the table part.

3. In a machine as claimed in claim 2, the ends of the screw havinghardened inserts therein engageable with the stop members.

4. In a machine as claimed in claim 1, the stop members comprisinghousings fixed to one of the bed and table parts having bores therein,stop parts threaded in said bores, and worm wheel operated means forturning said stop members.

5. In a machine as claimed in claim 4, said stop parts having domed endsfacing the part whose movement is limited thereby.

6. In a machine as claimed in claim 1, said adjustable stop membersincluding stop parts having domed ends facing the part whose movement islimited thereby.

7. In a machine as claimed in claim 1, said screw part being mounted onthe base part and said stop members being mounted on the table part andlimiting the movement of the nut part with respect to the table part.

8. In a machine as claimed in claim 1, said screw part being mounted onthe table part and said stop members being mounted on the base part forlimiting the movement of the nut part with respect to the base part.

9. In a machine as claimed in claim 1, bearings mounting the ends of thescrew part, the length of the screw part being substantially no greaterthan the sum of the length of the bearings, the connecting means to thegearing and the translational movement of the table part.

10. In a machine as claimed in claim 1, said adjustable stop memberseach comprising a stop part mounted for movement towards and from thepart whose movement it limits, a wedge operatively engageable with thestop part to produce movements towards such part, said wedge beingmounted for movement transversely with respect to the direction ofmovement of the stop part, and spring means acting on the wedge to moveit in one direction.

11. In a machine as claimed in claim 10, means to move the wedge in theother direction comprising a member engageable with the wedge, a fixedhousing, having a bore therein, said member being threaded in said bore,and worm wheel means to produce turning of the member.

12. In a machine as claimed in claim 1, each stop member havingassociated therewith a displacement pickup having a feeler memberengaging the stop member for sensing the displacement thereof, eachpickup being connected to an individual indicator for indicating theadjusted position of the associated stop, and both pickups beingconnected to a differential indicator for indicating any differentialoccurring between the adjusted positions of the stops. i i i i

1. In an intermittent tooth-generating grinding machine for gear wheelshaving a bed part and a table part and means mounting the table part fortranslational movement with respect to the bed part, a screw element,means mounting the screw element on one of said bed and table parts forturning movement with respect to such part, a nut element threadedlyengaged on said screw element, and a pair of adjustable stop membersmounted on one of the table and bed parts in the path of movement of oneof the screw and nut elements for limiting movement of such element withrespect to the part on which the stop members are mounted, one of thescrew and nut elements being fixed to one of the bed and table parts andthe other of the elements being movable between the stop members forengagement therewith, means fixedly holding the nut element nonrotatablyabout the axis of the screw element, whereby turning of the screwelement produces movement of one of the screw and nut elements when suchelement is out of engagement with the stop members without producingmovement of the table and produces movement of the table after suchengagement, a gearcase including gearing carried by one of the bed andtable parts, said screw element being located completely outside thegearcase, and means operatively connecting the screw element to thegearing to be driven thereby alternately in opposite directions, saidgearcase being mounted on the other of the bed and table parts from thaton which the stop members are mounted.
 2. In a machine as claimed inclaim 1, the screw part being mounted on the bed part for axial movementwith respect thereto, the stop members being mounted on the bed part andlimiting such movement of the screw part, and the nut part being fixedto the table part.
 3. IN a machine as claimed in claim 2, the ends ofthe screw having hardened inserts therein engageable with the stopmembers.
 4. In a machine as claimed in claim 1, the stop memberscomprising housings fixed to one of the bed and table parts having borestherein, stop parts threaded in said bores, and worm wheel operatedmeans for turning said stop members.
 5. In a machine as claimed in claim4, said stop parts having domed ends facing the part whose movement islimited thereby.
 6. In a machine as claimed in claim 1, said adjustablestop members including stop parts having domed ends facing the partwhose movement is limited thereby.
 7. In a machine as claimed in claim1, said screw part being mounted on the base part and said stop membersbeing mounted on the table part and limiting the movement of the nutpart with respect to the table part.
 8. In a machine as claimed in claim1, said screw part being mounted on the table part and said stop membersbeing mounted on the base part for limiting the movement of the nut partwith respect to the base part.
 9. In a machine as claimed in claim 1,bearings mounting the ends of the screw part, the length of the screwpart being substantially no greater than the sum of the length of thebearings, the connecting means to the gearing and the translationalmovement of the table part.
 10. In a machine as claimed in claim 1, saidadjustable stop members each comprising a stop part mounted for movementtowards and from the part whose movement it limits, a wedge operativelyengageable with the stop part to produce movements towards such part,said wedge being mounted for movement transversely with respect to thedirection of movement of the stop part, and spring means acting on thewedge to move it in one direction.
 11. In a machine as claimed in claim10, means to move the wedge in the other direction comprising a memberengageable with the wedge, a fixed housing having a bore therein, saidmember being threaded in said bore, and worm wheel means to produceturning of the member.
 12. In a machine as claimed in claim 1, each stopmember having associated therewith a displacement pickup having a feelermember engaging the stop member for sensing the displacement thereof,each pickup being connected to an individual indicator for indicatingthe adjusted position of the associated stop, and both pickups beingconnected to a differential indicator for indicating any differentialoccurring between the adjusted positions of the stops.